What does “mixtape” mean anymore? In assembling this list, we asked the question again and again, partly because rap continues to redefine the term. In 2000, mixtapes were generally still piles of exclusive freestyles sequenced by enterprising DJs like DJ Clue–they were promotional tools, pure and simple. In the middle of the last decade, after CD burners became cheap and ubiquitous, rappers like Lil Wayne began using them to build international audiences. And this year, Chance the Rapper released 14 sleek tracks with Kanye West and Lil Wayne cameos that he called a “mixtape,” and it debuted with the backing of Apple.

So there was only one hard-and-fast rule to make our cut: a “mixtape” was a free release, offered directly to rap fans. Other than that, everything else–whether it consisted of beats taken from other rappers’ albums or contained all original music, whether it was offered as promotion for a retail album or a standalone release–was up for grabs. The context of a release, both within an artist’s own career and within rap at large, mattered to us, as did replayability: The best mixtapes of the millennium double as some of the era’s best long-playing rap albums, because the distinction between the two has all but been erased. We hope you enjoy reading and debating as much as we did.